$maxN (aggregation accumulator)

Null and Missing ValuesNull和缺少值

• $maxN filters out null and missing values.筛选出空值和缺少的值。

Consider the following aggregation that returns the maximum n documents from a group:考虑以下聚合，该聚合从一个组中返回最多n个文档：

db.aggregate( [
   {
      $documents: [
         { playerId: "PlayerA", gameId: "G1", score: 1 },
         { playerId: "PlayerB", gameId: "G1", score: 2 },
         { playerId: "PlayerC", gameId: "G1", score: 3 },
         { playerId: "PlayerD", gameId: "G1" },
         { playerId: "PlayerE", gameId: "G1", score: null }
      ]
   },
   {
      $group:
      {
         _id: "$gameId",
         maximumThreeScores:
            {
               $maxN:
                  {
                     input: "$score",
                     n: 4
                  }
            }
      }
   }
] )

In this example:在此示例中：

• $documents creates the literal documents that contain player scores.创建包含玩家分数的文字文档。
• $group groups the documents by gameId. 按gameId对文档进行分组。This example has only one gameId, G1.这个例子只有一个gameId，G1。
• PlayerD has a missing score and PlayerE has a null score. 缺少score，而PlayerE的score为空。These values are both considered as null.这些值都被视为null。
• The maximumThreeScores field is specified as $maxN with input : "$score" and returned as an array.maximumThreeScores字段被指定为$maxN使用input : "$score"，并作为数组返回。
• Since there are only 3 documents with scores maxN returns the maximum 3 score fields even though n = 4.由于只有3个文档的scores为maxN，因此即使n=4，也会返回最多3个scores字段。

[
   {
   _id: 'G1',
   maximumThreeScores: [ 3, 2, 1 ]
   }
]

Comparison of $maxN and $topN Accumulators$maxN和$topN累加器的比较

Both $maxN and $topN accumulators can accomplish similar results.$maxN和$topN累加器都可以实现类似的结果。

In general:一般来说：

• $maxN has the advantage of finding maximum values in no particular sort order. 具有不按特定排序顺序查找最大值的优点。If you want to know the maximum values for n documents use $maxN.如果要知道n个文档的最大值，请使用$maxN。
• If guaranteing a particular sort order is a requirement use $topN.如果需要保证特定的排序顺序，请使用$topN。
• Use $topN if you don't intend on sorting on the output values.如果不打算对输出值进行排序，请使用$topN。

Window Function and Aggregation Expression Support窗口函数和聚合表达式支持

You can use $maxN as an accumulator.您可以使用$maxN作为累加器。

$maxN is supported as an aggregation expression.支持作为聚合表达式。

$maxN is supported as a window operator.支持作为窗口运算符。

Memory Limit Considerations内存限制注意事项

Aggregation pipelines which call $maxN are subject to the 100 MB limit. 调用$maxN的聚合管道受100 MB限制。If this limit is exceeded for an individual group, the aggregation fails with an error.如果单个组超过了此限制，则聚合将失败并返回错误。

Find the Maximum Three Scores for a Single Game查找单个游戏的最高三个分数

You can use the $maxN accumulator to find the maximum three scores in a single game.您可以使用$maxN累加器查找单个游戏中最多三个分数。

db.gamescores.aggregate( [
   {
      $match : { gameId : "G1" }
   },
   {
      $group:
         {
            _id: "$gameId",
            maxThreeScores:
               {
                  $maxN:
                  {
                     input: ["$score","$playerId"],
                     n:3
                  }
               }
         }
   }
] )

The example pipeline:示例管道：

• Uses $match to filter the results on a single gameId. In this case, G1.使用$match筛选单个gameId上的结果。在本例中为G1。
• Uses $group to group the results by gameId. In this case, G1.使用$group按gameId对结果进行分组。在本例中为G1。
• Specifies the fields that are input for $maxN with input : ["$score","$playerId"].使用input : ["$score","$playerId"]指定$maxN的输入字段。
• Uses $maxN to return the maximum three score elements for the G1 game with n : 3.使用$maxN返回n:3的G1游戏的最多三个分数元素。

The operation returns the following results:该操作返回以下结果：

[
   {
      _id: 'G1',
      maxThreeScores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
   }
]

Finding the Maximum Three Scores Across Multiple Games在多场比赛中找到最高三分

You can use the $maxN accumulator to find the maximum n scores in each game.您可以使用$maxN累加器查找每个游戏中的最大n个分数。

db.gamescores.aggregate( [
   {
      $group:
      {
         _id: "$gameId",
         maxScores:
            {
               $maxN:
                  {
                     input: ["$score","$playerId"],
                     n: 3
                  }
            }
      }
   }
] )

The example pipeline:示例管道：

• Uses $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Uses $maxN to return the maximum three score elements for each game with n: 3.使用$maxN返回n:3的每个游戏的最多三个分数元素。
• Specifies the fields that are input for $maxN with input: ["$score","$playerId"].使用input: ["$score","$playerId"]指定$maxN的输入字段。

The operation returns the following results:该操作返回以下结果：

[
   {
      _id: 'G1',
      maxScores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
   },
   {
      _id: 'G2',
      maxScores: [ [ 80, 'PlayerD' ], [ 66, 'PlayerC' ], [ 14, 'PlayerB' ] ]
   }
]

Computing n Based on the Group Key for $group基于$Group的组密钥计算n

You can also assign the value of n dynamically. 还可以动态指定n的值。In this example, the $cond expression is used on the gameId field.在本例中，$cond表达式用于gameId字段。

db.gamescores.aggregate([
   {
      $group:
      {
         _id: {"gameId": "$gameId"},
         gamescores:
            {
               $maxN:
                  {
                     input: ["$score","$playerId"],
                     n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } }
                  }
            }
      }
   }
] )

The example pipeline:示例管道：

• Uses $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Specifies the fields that input for $maxN with input : ["$score","$playerId"].指定使用input : ["$score","$playerId"]为$maxN输入的字段。
• If the gameId is G2 then n is 1, otherwise n is 3.如果gameId为G2，则n为1，否则n为3。

The operation returns the following results:该操作返回以下结果：

[
   { _id: { gameId: 'G2' }, gamescores: [ [ 80, 'PlayerD' ] ] },
   {
      _id: { gameId: 'G1' },
      gamescores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
   }
]